The growing need of high quality upper-air climate observations caused the GCOS (Global Climate Observing System) to initiate the establishment of a reference measurement network which is designed to provide climate data records not only at the surface of the Earth but throughout the atmospheric column into the stratosphere. Therefore GRUAN (GCOS Reference Upper Air Network) was envisaged by the WMO in 2007. The implementation meeting took place in February 2008 (see GCOS-121) at the Meteorological Observatory in Lindenberg (Germany), where the GRUAN Lead Centre was established. GRUAN is envisaged as a network of 30-40 observing stations. It builds on existing observational networks and operational capabilities and is expected to be complemented by ground-based state of the art instrumentation. The task of GRUAN is to provide long-term, highly accurate measurements of the atmospheric profile with emphasis to the upper troposphere and lower stratosphere (upper-air). The particular demand is to define and assure reference quality for the data and data products. The priority is on the Essential Climate Variables (ECV) defined by GCOS, which are temperature, wind speed and direction, water vapor, cloud properties, and Earth radiation budget (including solar irradiance). A high reliability of the data, which mandatorily includes e.g. well estimated uncertainties and therefore enables comparability and complementation of different techniques, is a prerequisite to fully characterize the properties of the atmospheric column and quantitatively encompass the climate change and the underlying causes, e.g. to separate the long-term change signal from the natural variability.
The definition of reference quality includes in a consistent way the full traceability of all measurements to SI units or internationally accepted standards, uncertainty analyses, which distinguish contributions from systematic and random error, comprehensive documentation, data validation, and metadata collection and management. With the quality level of the GRUAN data, results from more spatially-comprehensive global observing systems including satellites and current radiosonde networks can be constrained and calibrated.